Operational impact
If sent items are stored only in personal mailboxes, shared mailbox continuity breaks: the team cannot see what was sent, when, or by whom,
and troubleshooting becomes guesswork.
Why this happens
When a user sends as/on behalf of a shared mailbox, the message originates from a delegated context. Depending on the client and the permission model, Exchange Online can store the sent copy in the sender’s mailbox by default. This is not a bug; it is default behavior unless configured otherwise.
Understand the send model
- Send As: message appears from the shared mailbox identity.
- Send on Behalf: message appears as “User on behalf of Mailbox”.
You should enable sent item copy settings for both models if your operations require shared visibility.
Deterministic fix
Enable these mailbox properties on the shared mailbox:
# Connect-ExchangeOnline $SharedMailbox = "shared@domain.com" Set-Mailbox -Identity $SharedMailbox ` -MessageCopyForSentAsEnabled $true ` -MessageCopyForSendOnBehalfEnabled $true
Verification
- Send a test message using Send As from the shared mailbox.
- Send a test message using Send on Behalf (if used in your org).
- Confirm a copy appears in the shared mailbox Sent Items folder.
If it still does not behave as expected
- Confirm the send model used matches the setting you enabled (Send As vs Send on Behalf).
- Test in Outlook on the web to reduce client-side variability.
- Confirm the user is actually sending with the shared mailbox identity, not their own address.
- Check whether additional mailbox-level policies or add-ins are altering send behavior.